From News Coulomb.

So the big announcement finally happened, and GM electric vehicles now have access to the Tesla Superchargers. Overall, this is good news; however, after charging at two different Supercharger locations using the Tesla app and A2Z adapter, I felt that the experience was lacking.

The first Supercharger in Williams, CA was busy, making it almost impossible to access an open stall, the app was slow and buggy, and the cost to charge was extremely high. Based on my experiences, I think that the experience for non-Tesla EV owners on the Superchargers is going to depend a lot on the specific make and model of the EV and the location. Outside of California, I can imagine that Supercharger access will be far more important and a better overall experience for drivers of non-Tesla EVs.